The use of prostheses for the replacement of blood vessels and other anatomical ducts is of great interest in medicine and veterinary work. The use of biomaterials in prostheses and biomedical devices is reviewed in Hanker, J. S. et al., Science 242:885-892 (1988), and by Gebelein, C. G., "Prosthetic and Biomedical Devices" in Kirk-Othmer, Concise Encyclopedia of Chemical Technology, M. Grayson, ed., Wiley & Sons, 1985, pp. 965-968, both incorporated herein by reference. To be acceptable in a given application, a prosthesis must exhibit the proper mechanical properties and bio-acceptable composition for the given application. For example, vascular prostheses must provide a bio-acceptable surface which is conducive to cellular attachment and sustained blood flow, but yet is strong enough not to split or tear. Especially it is critical that the vascular prosthesis not tear along the body of the prosthesis or at the site of the sutures.
Collagen has been proposed as a biomaterial which has many properties desirable of a medical prosthesis. Collagen is a family of fibrous proteins that have been classified into a number of structurally and genetically distinct types (Stryer, L. Biochemistry, 2nd Edition, W. H. Freeman & Co., 1981, pp. 184-199).
Type I collagen is the most prevalent form. Type I collagen is found in skin, tendons, and bones and consists of two subunits of .alpha.1(I) collagen and one subunit of a different sequence termed .alpha.2. Other types of collagen have three identical subunits or chains, each consisting of about 1,000 amino acids.
Different tissues express different types of collagen, depending upon their structural needs. For example, type II collagen is found in cartilage, type III collagen is found in blood vessels and the cardiovascular system, and type IV collagen is localized in basement membranes. Collagen is a unique protein in that it forms insoluble fibers that have a high tensile strength.
Tubes made of pure collagen have been proposed for use as vascular prostheses (Noishiki, Y. et al., U.S. Pat. No. 4,690,973; Chu, G. U.S. Pat. No. 4,655,980; and Huc, A. J. Am. Leather Chem. Assoc. 80:195-212 (1985)); arterial prostheses (Maurer, P. et al., Eur. Surg. Res. Sep.-Oct., p. 90, (1983)); ureteral replacements, (Tachibana, M. et al., J. Urology 133:866-869 (1985)); and as a microencapsulation material for the oral administration or implantation of controlled release substances (Huc, A., et al., U.S. Pat. Nos. 4,711,783 and 4,670,014; Sanders, N. J., Chem. Eng. News, Apr. 1, 1985, p. 30-48).
However, the mechanical properties of prostheses made solely of collagen are not satisfactory for many purposes due to a tendency to tear or split. Especially, prostheses made solely of collagen are not satisfactory for replacement of vessels with a small diameter (Huc, A., J. Am. Leather Chem. Assoc. 80:195-212 (1985)). Attempts to strengthen the mechanical characteristics of collagen by fixing or tanning it have not been successful. Grillo found collagen tubes too weak to allow fine silk suturing without splitting at the sites of the needle puncture (Grillo, H. C. et al., J. Surg. Res II (1): 69-82 (1962)).
To avoid the mechanical deficiencies of collagen tubular prostheses, prostheses composed of other polymers such as Dacron and polyurethane have been prepared (Robert, A.-M. et al., Pathol. Biol. (Paris) 24-Supp.:42-47 (1976); Maupepit, P., EP Patent Application Publication No. 058623); as well as combinations of plastics and polyurethane (Hanson, S. R., U.S. Pat. No. 4,687,482; Mano, H. et al.. U.S. Pat. No. Re. 31,618; and Buddecke, E., DE Patent No. 1,494,939). However, these prostheses lack the biocompatability of collagen, and do not promote the revascularization of the prosthesis in the manner that collagen does (Huc, A., J. Am. Leather Chem. Assoc. 80: 195-212 (1985)). In addition, prostheses made solely from synthetic materials often evoke a foreign body response, suffer from fatigue or are potentially toxic or carcinogenic (Grillo, H. C. et al., J. Surg. Res. II (1):69-82 (1982)).
A composition comprising foam polyurethane and collagen has been used as a contraceptive sponge (Vorhaur, B., Biofluid Mechanics, vol. 2, 1980, Plenum, pp. 93-124) and to promote neovascularization (Lamberton, P. et al., ASAIO Abstracts 16:29 (1987)). Lamberton also proposed the use of a collagen impregnated polyurethane sponge to promote neovascularization in endocrine or hepatic transplantation, soft tissue prosthesis, bone graft or drug delivery systems (Lamberton, P. et al., ASAIO Abstracts 16:29 (1987)).
But there remains a need for a prosthesis which provides the bio-compatibility of collagen with the required pliancy and mechanical strength for use in medical applications such as conduits for the replacement of a missing, diseased or damaged biological vessel and especially a biological vessel with a small diameter.